It is now a well established principle of diabetes care that in order to prevent, delay, and/or reduce the complications of diabetes, it is desirable to maintain, as much of the time as possible, a blood glucose concentration (BGC) within or close to the normal range. Yet, at present, for individuals who must use exogenous insulin, the goal of maintaining euglycemia or near euglycemia a large fraction of the time is difficult or impossible. While subcutaneous insulin therapy empowers the patient to decrease BGC very effectively, the risk of hypoglycemia from too much exogenous insulin forces patients to frequently use too little insulin. As a result, most patients experience higher than normal average BGC with occasional episodes of both very high and very low BGC. Maintaining continuous euglycemia is challenging because it requires balancing the intensity of self-administered, subcutaneous insulin's action with the insulin action required to keep BGC steady, and to do so continuously on a timescale of less than one hour. Since several factors that can only be estimated impact this balance, even the most diligent and capable individuals using the best technology available cannot prevent their BGC from occasionally straying outside of the normal range. When testing reveals that BGC is too high or too low, corrective measures can be taken to reestablish euglycemia. Continuous blood glucose concentration data provides the fastest possible indication that corrective measures should be taken, and therefore, continuous glucose monitors were predicted to be particularly useful for optimizing blood glucose control.
In the first long-term study in which real-time, continuous blood glucose concentration data was available to insulin-using individuals with type I diabetes in the home setting (Diabetes Care, March 2004, pp. 734-738), blood glucose control was significantly better than when the same patients relied only on conventional, periodic blood glucose testing. With access to continuous data, patients spent 88% more time with their BGC in the 80-140 mg/dL (euglycemic) range, 47% less time with their BGC below 56 mg/dL, and 25% less time with their BGC above 239 mg/dL. However, even though blood glucose control was greatly improved by access to continuous blood glucose concentration data, these patients still spent only 9 hours per day with their BGC in the 80-140 mg/dL range, and they spent 7 hours per day with their BGC either below 56 mg/dL or above 239 mg/dL. Clearly, continuous blood glucose monitors can help improve blood glucose control, but access to continuous blood glucose concentration data alone is not sufficient to enable most patients to maintain euglycemia or near euglycemia as much of the time as would be ideal.
An important factor that limits blood glucose control and which is inherent to subcutaneous insulin therapy is the lag time between the delivery of subcutaneous insulin and its action. The fact that in order to maintain euglycemia after a meal, even the fastest insulin analogs should be dosed subcutaneously approximately 15 minutes before the meal, means that forethought is required to provide timely exogenous insulin action. Often, such forethought is not practical or is not exercised for other reasons, and a high BGC after a meal results. However, the need for forethought is not the only difficulty stemming from subcutaneous insulin's lag time. The combination of the lag time and the difficulty of knowing precisely how much insulin is required greatly complicates blood glucose control. The amount of exogenous insulin needed can only be estimated because it is a function of variables that typically are not precisely known—namely, insulin sensitivity, food quantity and composition, physical activity level, the amount of insulin already in the subcutaneous tissue and blood, and the blood concentrations of other hormones. Were it not for subcutaneous insulin's lag time, it would be theoretically possible to closely control BGC, even without knowing in advance exactly how much insulin is required, by repeatedly dosing small amounts of insulin subcutaneously when, and only when, continuous blood glucose data indicates a higher than desired BGC. In fact, a healthy pancreas, which delivers insulin directly into the bloodstream, and therefore with minimal lag time, in response to its own continuous blood glucose sensing, controls BGC in this manner, albeit aided by additional signaling pathways. However, the fact of the lag time of subcutaneous insulin action, means that even continuous glucose data cannot provide truly timely feedback on whether the amount of insulin already dosed is correct. Therefore, whether insulin is administered by injections or with an insulin pump, even with the benefit of a continuous glucose monitor, it is still desirable for the patient to develop the skill of gauging how much insulin to administer and when to administer it, and it is still the case that BGC will occasionally stray out of the normal range such that the patient will need to manage a quick and safe return to euglycemia.
Among the many, commonly encountered situations in which BGC has strayed or will stray outside of the target range are situations in which an insulin delivery problem has occurred, situations in which an insulin pump and continuous glucose monitor have been removed to participate in athletics, situations in which insulin has not been administered until after a child, who does not eat predictably, has eaten, situations in which a food that raises BGC rapidly has been eaten too soon after insulin administration, situations in which the carbohydrate content of a food has been underestimated or overestimated, situations in which an abnormally high BGC has been corrected with excessive insulin, situations in which the effect of exercise on BGC has been incorrectly estimated, situations in which stress has resulted in an unanticipated increase of BGC, and situations in which in the pharmacodynamic profile of insulin administered does not match optimal insulin action in the aftermath of a meal.
Embodiments of the invention described herein comprise new features of a continuous glucose monitor and methods for their use. These new features are designed to aid a user in managing situations in which BGC has strayed or will stray outside of the normal or target range and also to enhance the user's skill of predicting how BGC will respond to the various factors that affect it. Specifically, embodiments of the invention relate to notifications that a continuous glucose monitor provides to a user as a function of BGC and the criteria that trigger those notifications.